In one type of an image formation apparatus, first, a latent image formation unit forms an electrostatic latent image corresponding to a draft image on an image section of the photo sensor as an image holder. Then, the developing unit develops the electrostatic latent image formed on the image section of the photo sensor. Consequently, a toner image is prepared using a charged toner on the image section of the photo sensor. A transfer unit transfers the toner image formed on the image section of the photo sensor onto a transfer material or an intermediate transfer unit like paper or an OHP sheet as the unit that receives the image.
In the intermediate-transfer type image formation apparatus that uses an intermediate transfer unit as the image-receiving unit, it is possible to form a color image as is known well. In forming a color image in this intermediate transfer type image formation apparatus, first, the latent image formation unit sequentially forms electrostatic latent images, that are a draft image resolved into four colors, onto a photo sensor as an image holder. Next, the developing unit sequentially develops the electrostatic latent images of the four colors formed on the photo sensor, thereby to sequentially form color toner images of four-color charged toners of yellow, magenta, cyan, and black, on the photo sensor. An intermediate transfer process is carried out four times to transfer the toner images of the four colors formed on the photo sensor onto the intermediate transfer unit, thereby to sequentially superimpose the four-color toner images on the intermediate transfer unit to complete a primary transfer. Thereafter, the four full-color toner images obtained by the primary transfer based on the superimposition on the intermediate transfer unit are collectively transferred onto a transfer material like paper or an OHP sheet to complete a secondary transfer. As a result, a full-color image is formed on the transfer material. Various proposals have been made for the intermediate transfer unit. These include the units that use a resin belt having a sufficient lubricating surface like polyimide, PVDF, and ETFE, and a rubber material like urethane, NBR, and CR.
In another type of an image formation apparatus, transfer material wound around a transfer drum rotates in contact and in synchronism with the photo sensor. Based on this, toner images of various colors formed on the photo sensor are subsequentially transferred onto the image-receiving material wound around the transfer drum. According to this transfer drum type image formation apparatus, the transfer material used for this image formation apparatus is limited to the one that can be wound around the transfer drum. Therefore, there is a limitation to the use of the transfer material, as compared with the transfer material that is used in the intermediate transfer type image formation apparatus. Further, the transfer pressure applied at the time of transferring the image changes depending on the thickness of the transfer material. Therefore, this has a disadvantage in that color registration becomes unstable.
In the above image formation apparatuses, a reduction in sizes of these apparatuses and an increase in the image formation speed have been demanded in recent years.
However, reducing sizes and increasing the image formation speed of the apparatuses in order to satisfy these requirements has had the following difficulty. It is not possible to sufficiently develop an electrostatic latent image that is formed on the image section of the photo sensor, according to the general developing system, as described later. Further, when the rate of adhesion of the charged toner to the electrostatic latent image (the developing efficiency) is increased to compensate for the shortage in the development of the electrostatic latent image, the quantity of toner adhered to a non-image section (the texture section, or a section where there is no image) increases. The toner must not adhere to this section in principle. Consequently, what is called a “texture stain” phenomenon has occurred easily on the transfer image.
In order to sufficiently develop the image section of the electrostatic latent image formed on the photo sensor with the charged toner, it is usually necessary to form a development nip in the developing section between the photo sensor and the development roller, for example. This development nip has sizes that enable the securing of a developing time around 50 mm/sec to 100 mm/sec. Therefore, when the sizes of the apparatus are simply reduced or the image formation speed is increased, the sizes of the photo sensor and the development roller are reduced, and it becomes impossible to form a development nip having sufficient sizes. Further, the rotation speeds of the photo sensor and the development roller are increased, which makes it impossible to secure a sufficient developing time.
Therefore, when the sizes of the apparatus are simply reduced or the image formation speed is simply increased in the image formation apparatus, it becomes impossible to secure a sufficient developing time, and the development efficiency of the electrostatic latent image is lowered at the developing time. In order to compensate for a reduction in the development efficiency due to the reduction in sizes and increase in the speed of the image formation apparatus, there has been the following method. This method is to increase the quantity of toner adhesion to the electrostatic latent image formed on the photo sensor by increasing the development bias. According to this method, the efficiency of developing the image section of the photo sensor improves. However, the quantity of the toner adhered to the non-image section of the photo sensor increases, and this generates the “texture stain” on the transfer image.
Further, there is a method of using two development rollers which improve the development efficiency of the electrostatic latent image without changing the development bias. According to this method, however, it is necessary to prepare a new development roller, which leads to a cost increase. Further, installation space for this development roller is additionally required, which results in an increase in the sizes of the apparatus.